1. Field of the Invention
This invention relates generally to the mass storage disk drive media, and more particularly to the processing of head and sector numbers which address defective sectors.
2. Description of the Prior Art
Mass storage disk devices include a number of disks rotating about a common vertical axis. Both the top and bottom surfaces of each disk are typically coated with a magnetic material which retains magnetic bits. One read/write head is mounted on a carriage which moves in a radial and perpendicular manner to the common axis. Magnetic bits are written on the media surface when the carriage has positioned itself. Each head, therefore, would write in turn on the respective surfaces. The magnetic bit patterns for a surface are in concentric circles called tracks. The same track number on each surface represents a cylinder. The magnetic bit patterns are written in zones called sectors. Typically each track could be divided into 42 sectors. Therefore, to select a sector of data (magnetic bits), the disk device requires a cylinder number, a head number and a sector number. Data bytes are transferred between the disk device and a main memory, a sector at a time.
Typical disk drives store data in the order of megabytes: 24MB, 48MB, 100MB 500MB, etc. It is unreasonable to expect and uneconomical to build disk drives having surfaces which can record in every bit position and on every track. Accordingly, rules are negotiated between manufacturers and users that a number of defective sectors will be acceptable and the defective areas will be written in a predetermined track and sector.
This presents the problem of how does a user handle the defective sector in the normal computer operation without sacrificing throughput. Another requirement is for the user to designate defective sectors found during normal computer processing. A disk, therefore, could store both software-noted as well as vendor-noted defective sectors.
One obvious method is to compare the desired sector with the defective sector written in the defective sector track. This technique requires two seeks per data transfer thereby reducing system throughput.
U.S. Pat. No. 4,434,487 entitled "Disk Format for Secondary Storage" describes a system whereby an evenly distributed portion of each disk is reserved as spare sectors for replacing defective sectors. Also, there are stored on the disk multiple copies of a table containing a list of each replacement block and the address of any bad block mapped to it.
This table is searched to find the appropriate replacement address. These techniques reduce the number of sectors available for the storage of data bytes and require considerable software or firmware overhead to process the defective sector seeks.
U.S. Pat. No. 4,214,280 entitled "Method and Apparatus for Recording Data Without Recording on Defective Areas of a Data Recording Medium" describes a method which includes detecting a defective area on a data recording area, writing an address of the defective area on a data recording area, recording data on the data recording area by detecting the defective area with the address and writing part of the data on the data recording area up to the defective area and writing the remaining data on the data recording area succeeding the defective area.
U.S. Pat. No. 4,405,952 entitled "Apparatus for Detecting Faulty Sectors in a Magnetic Disk Memory" describes a system whereby one or more sectors on each track is a salvage sector. Here again the storage capacity of the disk is reduced.